diyAudio

diyAudio (http://www.diyaudio.com/forums/)
-   Pass Labs (http://www.diyaudio.com/forums/pass-labs/)
-   -   Newbie Class-A Build (http://www.diyaudio.com/forums/pass-labs/154472-newbie-class-build.html)

Wizardmaxx 5th November 2009 03:53 AM

Newbie Class-A Build
 
Hi There

I have built a couple of discrete and chip amps so far with success, but felt it time to delve into some Class-A.

The decision is which one. I have read through some of the forum posts and as a beginner I though I would go for the Aleph X, but that may not be the best starting point hence my choice of two candidates.

The F-5 or the Aleph-J due to their simplicity and move on to something more exotic later.

Now for the questions:

-Which one sounds better
-Which is the easiest to build
-Which is the cheapest to build
-Can they be bridged for higher output
-Can they be cascaded with additional output drivers

I know these are newbie questions, but you gotta start somewhere.

My partslist so far if it makes any difference

2x 300VA 18-0-18 Toroidals
2x 475VA 21-0-21 (DC) SMPS
670000uF of capacitors
Enough watercooling kit for around 600W at ambient plus 10 degrees

One last question - Is it possible to drive Hexfets with an opamp (the likes of an TL072). It was a concept Ii was toying with about 15 years back. I will post a design later if I can find the stone tablet I carved it out on.

All help and advice appreciated

Wizard

MEGA_amp 5th November 2009 04:59 AM

It's a subjective toss-up, having built F5 and variations of Aleph (I've yet to fire up my built AlephJ version..currently listening to SOZ...don't want to dismantle it just yet), I personally have an affinity for Aleph. You already have the expensive parts taken care of, 18V 300VA transformers per channel are the correct voltage/size to run either. The AlephJ uses either 1x 2SJ109 or 2x 2SJ74 for inputs, both discontinued, but still available. F5; 2SK170 (discontinued) and 2SJ74 for inputs, again both are still attainable. F5 can be bridged for more current delivery, which you may or may not need. A bridged AlephJ is essentially an AlephJ-X. And I do believe both versions can have their inputs cascaded if you have higher voltage requirements. AlephJ has minimally higher parts count, you've got the hard parts taken care of, I'd say build them both. You definitely won't be disappointed with either. As for your last question, from what I've learned, yes you can, but I haven't a clue, if you post a schematic or a pic of that stone tablet, I'm sure someone will help.


-john

Wizardmaxx 5th November 2009 06:03 AM

Thank for the reply Mega

Are there equivalent replacements for the 2SJ109 2SJ74 2SK170 & 2SJ74 that are suitable or do they have to be used.

Also when I meant cascaded I was referring to the concept below with more fet's in parallel for higher current...or am I being a numpty.

The concept I was playing on goes something like this. I know it is incorrect in many aspects, but it was all I could CAD up since my post. Criticism and/or ideas welcome. I know it is an AB amp, but I was thinking with enough bias and a decoupling cap it could operate as a class a. Either that or have a single rail suplly and run it in bridge mode. Again I am a class a newbie....sorry.

[IMG]http://img193.imageshack.us/img193/6837/123zw.png[/IMG]

If the pic doesn't show please someone tell me how.

Direct link to schematic

Imageshack - 123zw

Melon Head 5th November 2009 10:19 AM

Most people here try to avoid opamps, if they can.
So while it might work, it may not be the best solution.
What type of speakers do you have ie impedancy, efficiency?

Wizardmaxx 5th November 2009 01:23 PM

Hi thanh1973

I can understand the want to avoid opamps, but on paper they have great thd performance and are easy to use...in my opinion anyway.

As far as speakers go, nothing to talk about. Incarnations I have put together myself over the years. Lets say freq response of 100hz to 10000hz at an spl of 60db per w...if you know what I mean.

The beauty of the idea is scalability if the opamp can handle the extra fets. The original had a much smaller component count and at the time seemed a good idea.

What is the reason for aversion to opamps in general. I always had them in high regards due to paper specs.

carpenter 5th November 2009 02:24 PM

Quote:

Originally Posted by Wizardmaxx (Post 1971043)
Hi thanh1973

I can understand the want to avoid opamps, but on paper they have great thd performance and are easy to use...in my opinion anyway.

As far as speakers go, nothing to talk about. Incarnations I have put together myself over the years. Lets say freq response of 100hz to 10000hz at an spl of 60db per w...if you know what I mean.

The beauty of the idea is scalability if the opamp can handle the extra fets. The original had a much smaller component count and at the time seemed a good idea.

What is the reason for aversion to opamps in general. I always had them in high regards due to paper specs.

It's not that op-amps are bad...

My personal reason for avoiding op amps is their lack of ZEN. They may make the designer's life easier, but they are anything but simple on the inside. Most of the amps in the Pass forum are of a simple nature, much like your cascade--minus op-amp, and, philosophically (some would argue audibly), desirable.

Wizardmaxx 5th November 2009 03:00 PM

I will build the simplest class a possible with op amp and thanks for the insight into the simplistic concept carpenter (makes perfect sense), but thought one component to do most of the leg work was the way forward.

Now back to the Zen approach and my initial post.

Any other feedback on the questions asked as I will be starting the parts order very soon.

Also what power supply capacitance would suffice...as I have posted I have access to virtually unlimited uF and can somebody let me know what is the best way to 'charge' them up especially if it is very high capacitance. I once designed a slow charge circuit that kicked in a relay to bypass a limiting resistor in series, but is there a better way?

Thanks again

Wizard

carpenter 5th November 2009 03:19 PM

To control voltage/current inrush, Nelson taught us a neat trick using a thermistor (I think that's what it's called). Check out his tutorials and his Aleph amps.

DougL 5th November 2009 03:20 PM

Quote:

Originally Posted by Wizardmaxx (Post 1971113)
I will build the simplest class a possible with op amp and thanks for the insight into the simplistic concept carpenter (makes perfect sense), but thought one component to do most of the leg work was the way forward.

Now back to the Zen approach and my initial post.

Any other feedback on the questions asked as I will be starting the parts order very soon.

Also what power supply capacitance would suffice...as I have posted I have access to virtually unlimited uF and can somebody let me know what is the best way to 'charge' them up especially if it is very high capacitance. I once designed a slow charge circuit that kicked in a relay to bypass a limiting resistor in series, but is there a better way?

Thanks again

Wizard

Op Amps make me shudder. I can't get past the 741, which was "perfect"in the day. Op Amps have 60 or more db of open loop gain and 3 or more gain stages. The 741 started to roll off at 10 Hz. :confused: There are much better op amps (or so I have been told). To me, an op amp front end would be easy to do, but very hard to do right. I think a low parts count discrete front end is the way forward.

As far as capacitance, it depends on the size of the amp. For 25 Watt class A, look at the F5 and make the caps on the same order of magnitude and use the thermistors.

JPeitzman 5th November 2009 05:23 PM

Quote:

What is the reason for aversion to opamps in general
Quote:

on paper they have great thd performance
Quote:

due to paper specs
Hit the nail on the head :smash:

I have a few LM3875's collecting dust, they do get pulled out from time to time, but mostly to be dusted. I also have an LM4562 based line amp, it is a fairly highly regarded op amp, I sampled them from National back when they first came out and where near imposable to get. Let's just say that it was added to the dust collecting pile after about 5min of listening to a JFET Bride of Zen. The op amps may be salvaged for a cheap phono that I can through in my pocket, or they may be forever banished to my "glad I tried that" dust pile.

:cheers:
James


All times are GMT. The time now is 10:00 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio


Content Relevant URLs by vBSEO 3.3.2